Modern chemical detectors, such as artificial (electronic) noses, ion-mobility spectrometers, gas chromatographs and the like, have evolved such that miniaturized and hand-held, briefcase-sized-or-smaller chemical trace detectors are now available. If chemical signals are thoroughly dispersed in the atmosphere (e.g. nitrogen compounds in city smog), the application of a small suction at a device inlet can be sufficient to bring chemical traces to bear upon the sensor, thus affording the possibility of a detection step. However, many other cases exist where aerodynamic sampling is required before detection can occur. Canines, for example, are natural chemical trace detectors with a built-in aerodynamic sampler, the slit canine nostril [1], that is positioned in proximity to a trace chemical source with sampling and subsequent detection occurring, or that samples chemical plumes carried by the natural wind. Similarly, an active, air-moving sampler is required to “reach out” from a manmade hand-held or otherwise mobile detector in order to acquire vapor and/or particulate traces from surfaces being sampled.
There has been a variety of attempts to provide aerodynamic samplers. The potential-flow suction inlet is well known in fluid dynamics with the application of that science to heating, ventilation and air conditioning documented in many textbooks, e.g. [2]. The potential-flow suction inlet can take on several forms such as a blank tube, flanged tube, bellmouth inlet, etc. However, the “reach” of the potential-flow suction inlet is severely limited by the nature of potential flow. To overcome this limitation, scenting animals have developed long noses and the mobility to position them in close proximity to a scent source [1].
Another approach to aerodynamic sampling uses an intake vortex. Helmholtz's vortex laws reveal that a line vortex cannot end in free air, but it can attach to a solid surface. For example, jet engines can “suck up” rubble from runways through vortex impingement [3] and a tornado represents a vortex tube that attaches to the ground and extends powerful suction due to the low pressure in the vortex core. The vortex concept has been disclosed in relation to a sampling device with a small tornado-like swirling flow that may “reach out” to a surface and convey vapor/particulate traces from the surface to a sensor element of a trace detector [4]. The upward axial flow along the vortex core transports a trace sample to the inlet of the device, where suction applied to a small central tube captures some of the trace-bearing airstream. Thereafter, the trace-bearing airstream can be interrogated for chemical content by a suitable trace detector such as an ion mobility spectrometer (IMS).